1. Field of the Invention
This invention relates generally to the recovery of trivalent thallium values from solutions, and more specifically to the extractive recovery of trivalent thallium values from aqueous media.
2. Description of the Prior Art
Trivalent thallium compounds, i.e., thallic compounds, have been used as oxidizing agents in various reactions. For example, Kruse et al. J. Org. Chem. 36, 1154 (1971) describes the epoxidation of certain olefins with thallic acetate and U.S. Pat. No. 3,641,067 (issued in 1972 to W. Kruse) relates to the preparation of the epoxides of propylene and isobutylene by means of lower thallic alkanoates.
In all of these reactions the trivalent thallium is reduced to the monovalent state and, if the thallium is to be reused in the reaction, it is necessary to reoxidize or "regenerate" it by converting thallium (I) to thallium (III). Various methods for effecting this conversion have been proposed. Thus, it is proposed to convert thallium (I) to thallium (III) in the application of William Brill, entitled "Catalytic Conversion of Thallium (I) to Thallium (III), " Ser. No. 789,053, filed Apr. 21, 1977, by means of molecular oxygen using a Group VIII noble metal as a catalyst. In addition, it has been proposed in the application of Richard A. Johnson entitled, "Conversion of Monovalent Thallium to Trivalent Thallium," Ser. No. 740,147, filed Nov. 8, 1976, to convert thallium (I) to thallium (III) by means of molecular oxygen in the presence of a Group VIII noble metal catalyst and in the presence of a promoter comprising an alkali metal compound. The processes of both Ser. No. 789,053 and Ser. No. 740,147 typically result in an aqueous medium containing the desired trivalent thallium compound together with unconverted monovalent thallium compound and alkali metal compound, where the latter is employed as promoter in accordance with Ser. No. 740,147. It is generally desirable to recover the trivalent thallium compound from the aqueous medium produced by the foregoing processes to avoid passing monovalent thallium compound and alkali metal compound (where employed) as impurities to the subsequent reaction steps using the trivalent thallium compound. However, separation of the trivalent thallium compound from the aqueous medium by precipitation entails additional process steps which, albeit effective, introduce complexity into the recovery of the trivalent material, and evaporation of water from the aqueous medium containing the trivalent thallium compound does not achieve any separation of the trivalent thallium compounds from other salts contained in the aqueous medium.
It has been proposed in Japanese Patent Publication No. 50/92,296, published July 23, 1975, to extract thallic salts from aqueous solutions containing HBr or HCl using polar organic solvents and mixing the resulting organic layer with nonpolar organic solvents and water whereby the thallic ions are reextracted into the aqueous phase as the halogen thallium acid (e.g., HTlBr.sub.4 or HTlCl.sub.4). However, such a process is not readily adaptable to treatment of aqueous media containing monovalent and trivalent thallium values since the monovalent thallium ions will be precipitated as the corresponding halide, which precipitate must be recovered and treated for conversion of the monovalent thallium content thereof to a water-soluble form before further processing can be effected, e.g., before recycle of the monovalent thallium values to an oxidation step in which trivalent thallium values are formed therefrom.
Recovery of trivalent thallium values in the form of TlCl.sub.4.sup.- or TlBr.sub.4.sup.- using an anion exchange resin as suggested in U.S. Pat. No. 3,399,956 (issued in 1968 to I. Hirose et al.) is disadvantageous due to the high cost of the resin required for recovery of large amounts of trivalent thallium in an industrial scale process.
Other extraction techniques have been developed as analytical tools in which trivalent thallium ions are recovered from aqueous solutions employing such extractants as diethyldithiocarbamate and 8-quinolinol-4-thenoyltrifluoroacetone, as outlined in G. H. Morrison and H. Freiser, Solvent Extraction in Analytical Chemistry 237 (John Wiley & Sons 1957). However, such analytical extraction methods are not readily adaptable to industrial processes and offer the disadvantage of the extreme high cost of such extractants on an industrial scale.
Other extraction methods employed for treatment of liquids containing monovalent thallium are not readily adaptable to separation of trivalent thallium from aqueous mixtures containing monovalent thallium. See U.S. Pat. No. 4,031,196 (issued in 1977 to J. J. Leonard) (extraction of isobutyric acid with dibutyl ether from aqueous medium also containing barium salt and thallous isobutyrate) and A. Letheridge et al., J. Chem. Soc. Perkins I, p. 2763 (1973) (extraction of aqueous mixture containing thallous trifluoroacetate and octene-1 oxidation products with ether). However, the aqueous media treated in these references, did not contain trivalent thallium.